Airways (i.e. the trachea, bronchi, and bronchioles) that transport oxygen to the lungs for gas exchange are essential to human life. The largest airways of mice, and all human conducting airways, are lined with a pseudostratified epithelium made up of roughly equal numbers of secretory (Clara), ciliated, and basal cells and sparse neuroendocrine cells. The abundance, distribution, and function of these epithelial cells are disrupted in debilitating lung diseases such as chronic asthma, cystic fibrosis, and cancer. Recent evidence from our lab has shown that basal cells self-renew and generate differentiated Clara and ciliated cells over 14 weeks in vivo and for at least three passages in vitro. Therefore, as a progenitor population, basal cells have the potential to regulate the composition of the whole respiratory epithelium. Moreover, the identification of a population capable of long-term self-renewal and differentiation is critical for the development of cell-based therapies for lung diseases. By combining long-term genetic lineage tracing with confocal microscopy and collaborations with stem cell experts, Specific Aim 1 of this proposal will test the hypothesis that a subset of basal cells, residing in a highly vascularized niche, self-renews over very long periods. In Specific Aim 2, a novel assay will be used to test the hypothesis that components of the basal cell niche modulate their cell biology, self-renewal, and differentiation. Preliminary data has shown that IL13, a cytokine known to alter epithelial structure and function in chronic asthma, affects numerous basal cell behaviors in this assay in the absence of other cell types (e.g. stroma, immune cells). We will exploit this assay and transgenic mouse lines to test the functions of Bmp signaling in the maintenance of airway epithelial homeostasis. In the longer term, this assay will be used to identify novel effectors of basal cell behaviors and these findings will be related back to life using mouse models. In addition to providing multidisciplinary training in cell biology, lung (patho) biology, and stem cell biology, these data will throw new light on the contributions of progenitor cell behaviors to health and disease.